The present invention relates generally to electromagnetic transducers such as those used in audio speaker systems, and more particularly to an electromagnetic audio transducer with a lever diaphragm.
An electromagnetic audio transducer is a device used to create sound in speaker systems. FIG. 1 illustrates a cross-section view of a conventional cone style electromagnetic audio transducer known as a speaker. The speaker 10 includes a round supporting frame or basket 14, a round conical diaphragm or cone 18, a conductive coil of wire known as a voice coil 22 that is wound around a former 26, and a round magnetic system 30. The magnetic system 30 includes a donut-shaped permanent magnet 38 with opposite poles positioned between top and bottom flux conducting plates 42 and 46. The speaker 10 further includes a flux conductive pole piece 50 that is either part of, or connected to, the bottom plate 46. The top plate 42 and pole piece 50 define a gap 34 therebetween. The gap 34 is a low permeability air gap in the flux path of a magnetic circuit. The pole piece 50 directs and concentrates magnetic flux 36 across the gap 34. The voice coil 22 and the former 26 are attached to the cone 18, and the cone 18 is suspended from the basket 14 by a flexible surround 51 and spider 54. The flexible surround 51 and spider 54 center the voice coil 22 in the gap 34 where the lines of magnetic flux are concentrated. The voice coil 22 is thus positioned to reciprocate specifically along an axis 40 perpendicular to the lines of magnetic flux 36 in the gap 34.
The electromagnetic audio transducer, speaker 10, is defined by the cone 18, voice coil 22, former 26, surround 51, spider 54, basket 14, and magnet system 30. An actuator comprised of a magnet system 30 and voice coil 22 define the driver of the electromagnetic audio transducer of speaker 10. In operation, the speaker 10 is mounted to an enclosure called a speaker box (not shown), and the electrically conductive voice coil 22 receives an alternating current from an audio amplifier (not shown). The electrically charged or energized voice coil 22 in turn produces a dynamic electromagnetic field that reacts with the magnetic flux 36 in the gap 34 to create a reciprocating axial driving force in the voice coil 22 such that the voice coil 22 moves up and down in the gap 34 along the axis 40 in the directions of arrows A and B. Thus, the voice coil 22, former 26, and cone 18 reciprocate as one unit relative to the speaker box displacing air to create pressure waves in air identified as sound waves.
It is common for a speaker box to have more than one speaker to form a speaker system such that the two or more speakers, each producing sound within a different range of frequencies, will be radiated away from the box completing a full range of sound in the audible sound spectrum. Most commonly, these individual speakers are known as high, mid, bass, and sub-bass. The speakers for the bass and sub-bass frequencies need to move excessively larger volumes of air to produce their low frequencies in order to maintain a sound pressure level (SPL) consistently matched with the mid and high frequency speakers.
One way to displace larger volumes of air is to increase the axial movement of the cone 18. However, the axial movement of the cone 18 is mechanically limited by the suspension system of the surround 51 and spider 54 and by the limited range of movement of the voice coil 22 within the driver. The cone 18 of the speaker 10 will move to maintain a consistent SPL with the higher frequency speakers in the speaker system up to the point where one of the mechanical limitations has been reached. However, any axial movement beyond this point will result in a decline in sound quality. The decline in sound quality is known as distortion. Distortion occurs when sound output from the speaker 10 does not identically correspond to the electrical input signal to the speaker and results in poor sound quality. Furthermore, a decline or “rolling off” of the sound pressure level occurs below this point because the cone 18 is fixed in size and cannot displace the increased volume of air required by the lower frequencies.
Another problem with conventional audio speakers is that they are not efficient. Efficiency is expressed in terms of watts and is a percentage that is derived from the ratio of electrical input power applied to the speaker to the acoustical power output transmitted from the speaker. The typical efficiencies of modern audio speakers are in the range of only a few percent. Most of the electrical output from an audio amplifier is wasted by the speaker and dissipated off in the form of heat, not sound. Thus, speaker inefficiency can be very expensive and is a significant consideration in speaker design.
The speaker 10 of FIG. 1 has an “underhung” voice coil geometry where the voice coil 22 is shorter than the depth of the gap 34. The underhung voice coil 22 is not receiving an electrical input signal and thus is illustrated at its rest position. When a positive electrical input signal is applied to a positive terminal (not shown) on the speaker 10, the voice coil 22 and cone 18 move in the direction of arrow B toward a position of “cone extension.” Conversely, when a negative electrical input signal is applied to the same terminal on the speaker 10, the voice coil 22 and the cone 18 move in the direction of arrow A toward a position of “cone retraction.” FIG. 2 illustrates the speaker 10 of FIG. 1 where the cone 18 and voice coil 22 have moved to a position of cone extension. At this position, the voice coil 22 reaches an outer edge 33 of the gap 34, which is known as the maximum linear excursion (“Xmax”) position of the voice coil 22. When the cone 18 moves in the opposite direction to the cone retraction position, the voice coil 22 reaches an inner edge of the gap 34 and is in an opposite Xmax position. The full range of motion traveled by the voice coil 22 from an extended Xmax to a retracted Xmax is known as the speakers Xmax peak-to-peak parameter. When the voice coil 22 of the speaker 10 is not energized as illustrated in FIG. 1, the suspension system (the surround 51 and spider 54) will return the coil 22 to its rest position midway between the Xmax peaks. When the voice coil 22 is energized at sufficient energy levels and particularly at low frequencies, it will reciprocate past the Xmax peak-to-peak positions, temporarily moving and operating partially out of the gap 34. The voice coil 22 is then no longer moving linearly with the electrical input signal because a portion of the voice coil 22 is not within the gap 34 and not reacting with the magnetic field and thus the output sound signal will be distorted. The efficiency of the speaker 10 will also be reduced when the voice coil 22 operates beyond its Xmax positions because the electrical input power is not producing as much force and is dissipated as heat when the voice coil 22 is outside the gap 34.
The underhung voice coil geometry of speaker 10 maintains low distortion when operated within its Xmax range. The speaker 10 is relatively efficient as long as the voice coil 22 is operated within the Xmax range and thus within the magnetic field in the gap 34. The underhung speaker 10, however, is easily driven to operate beyond the Xmax by trying to produce very low frequencies or by over-powering the voice coil 22 to produce higher sound intensity levels. Over powering will not only cause the voice coil 22 to be driven beyond its Xmax range and distort the sound, it will also cause the voice coil 22 of the speaker 10 to quickly reach its thermal limit and overheat. Thus, the underhung voice coil geometry of speaker 10 in FIG. 1 is not able to produce undistorted high sound intensity levels at a lower frequency range and is better suited for higher efficiencies and lower distortion at the upper ranges of its bass frequencies.
The underhung voice coil geometry of speaker 10 of FIG. 1 can be modified to produce higher sound intensity levels at lower frequencies by using a larger top plate 42 and a correspondingly taller pole piece 50 to define a deeper gap 34 in which the voice coil 22 may travel further before reaching Xmax peak-to-peak. However, this “highly underhung” voice coil geometry can be less efficient than a standard underhung arrangement because the flux 36 (FIG. 1) in the gap 34 will not be as strongly concentrated due to the increase in surface area of the top plate 42.
FIG. 3 illustrates another conventional speaker 10a designed to overcome some of the drawbacks of the underhung speaker 10 (FIG. 1). The speaker 10a has an “overhung” voice coil geometry that extends out beyond the gap 34a from both ends when the voice coil 22a is at rest. The top plate 42a, and thus the gap 34a, is thin like that found in the underhung speaker 10 of FIG. 1 so that the flux 36a density is highly concentrated. As with the speaker 10 of FIG. 1, the speaker 10a moves in the direction of arrow B to cone extension or in the direction of arrow A to cone retraction depending on the polarity of the electrical input signal.
FIG. 4 illustrates the speaker 10a of FIG. 3 where the cone 18a has moved to the cone extension position and the voice coil 22a has moved to an Xmax in the direction of arrow B from the rest position. At this Xmax position, an inner edge of the voice coil 22a reaches an inner edge of the gap 34a. When the cone 18a moves in the opposite direction to the cone retraction position, the voice coil 22a moves in the direction of arrow A to an Xmax position past the rest position to where an outer edge of the voice coil 22a reaches an outer edge of the gap 34a. The voice coil 22a can move further along the axis 40a than can the underhung voice coil 22 in speaker 10 of FIG. 1 and thus produce a higher SPL at lower frequencies before distortion occurs. The larger voice coil 22a can also handle larger amounts of power. However, the voice coil 22a can be less efficient because a portion of the voice coil 22a is always operating outside of the gap 34a and thus wasting power. Furthermore, the larger size and mass of the voice coil 22a increases the opposing inertial forces acting on it such that the cone 18a cannot move as efficiently or fast to produce the higher frequencies as it could with the smaller voice coil 22 of the underhung speaker 10 (FIG. 1). Thus, a reduction in the efficiency in the upper range of bass frequencies may occur.
Conventional cone style speakers have another drawback when multiple speakers, each producing a different range of frequencies, are combined together within a single controlled space, such as a horn, to create a full range speaker system. Examples of such speaker systems are disclosed in U.S. Pat. Nos. 5,526,456 and 6,411,718. Because of the irregular shape of their conical diaphragms (the speaker cone), the low and mid frequency transducers in this type of speaker system positioned in the walls of the horn disrupt the paths of the higher frequencies produced by the high frequency transducers near the apex of the horn. In order to prevent the conical diaphragms from disrupting the paths of the higher frequencies, special adapters and apertures are added to the horn to maintain the continuity of the horn wall. Also, the round periphery of a conical diaphragm does not maximize use of the available horn wall area upon which it is mounted and thus wastes useful horn wall space.
Therefore, a need exists for a transducer for use in an audio speaker system that is capable of producing high sound intensity levels while maintaining high electrical efficiencies and low distortion and that may be combined with other audio transducers in a speaker system such that it can provide continuity in the wall of a horn and a low disruptive path for the sound waves emitted by the other audio transducers within the speaker system.